Propylene oxidation mechanisms and intermediates using in situ soft X-ray fluorescence methods on the Pt(111) surface

The oxidation of propylene preadsorbed on the Pt(111) surface has been char acterized in oxygen pressures up to 0.02 Torr using fluorescence yield near -edge spectroscopy (FYNES) and temperature-programmed fluorescence yield ne ar-edge spectroscopy (TP-FYNES) above the carbon K edge. During oxidation o f adsorbed propylene, a stable intermediate was observed and characterized using these soft X-ray methods. A general in situ method for determining th e stoichiometry of carbon-containing reaction intermediate species has been developed and demonstrated for the first time. Total carbon concentration measured during temperature-programmed reaction studies clearly indicates a reaction intermediate is formed in the 300 K temperature range with a surf ace concentration of 0.55 x 10(15) carbon atoms/cm(2). By comparing the int ensity of the C-H o* resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate can be determined unambiguously. Based on calibration with molecular propylene (C3H6) and pr opylidyne (C3H5), the intermediate has a C3H5 stoichiometry for oxygen pres sures up to 0.02 Torr. A set of normal and glancing angle FYNES spectra abo ve the carbon K edge was used to characterize the bonding and structure of this intermediate. Spectra of known coverages of adsorbed propylene and pro pylidyne served as standards. The spectra of di-sigma propylene, propylidyn e. and the intermediate were curve fit as a group with consistent energies and widths of all primary features. Based on this procedure, the intermedia te is 1,1,2-tri-sigma 1-methylvinyl. The stoichiometry and temperature stab ility range of the 1-methylvinyl intermediate formed in oxygen pressures up to 0.02 Torr is identical with the stoichiometry and stability of the same intermediate formed during oxidation of preadsorbed propylene by excess co adsorbed atomic oxygen.